Question

When I add slow code to the OnChange event of TPageControl I run into problems.

If the code is fast and doesn’t take a lot of time, things are fine.
However if the code takes a long time to return +/- 0.5 to 1 second, the PageControl starts to act weird.

If the user changes a page sometimes it doesn’t do anything on the first click, and a second click on the page is required to actually make the change happen.

I’ve kind of sort of fixed this with code like this.
(I’ve simplified it a bit, just to show the idea)

type TDelayProc = procedure(Sender: TObject) of object;

TForm = class(TForm)
...
private
  FDelayedSender: TObject;
  FDelayedEvent: TDelayProc;
  procedure SetDelayedEvent(Value: TDelayProc);
  property FDelayedSender: TObject read FDelayedSender write FDelayedSender;
  property FDelayedEvent: TDelayProc read FDelayedEvent write SetDelayedEvent;
...

procedure TForm1.SetDelayedEvent(Value: TDelayProc);
begin
  Timer1.Active:= false;
  FDelayedEvent:= Value;
  if Assigned(Value) then Timer1.Active:= true
  else DelayedSender:= nil;    
end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  Timer1.Active:= false;
  if Assigned(DelayedEvent) then DelayedEvent(DelayedSender);
end;

procedure TForm1.PageControl1Change(Sender: TObject);
begin
  if PageControl1.ActivePage = TSPage1 then begin
    DelayedSender:= Button1;
    DelayedEvent:= Button1Click;
  end; {if}
end;

As you can see this is a horrible hack.
The code I’m calling is in QuickReport to prepare a report and MySQL query and such, so I don’t have much control over that.

I’m think there’s some Win32 messaging that I’m messing up by not returning from TPageControl.OnChange fast enough, the delay is definitely shorter than 3 seconds though.

I’ve tried ProcessMessages, but that just made things worse and I don’t want to use a separate thread for this.

How do I fix this so I can use the OnChange event handler like normal

Answer 1

I’m unclear about why you’re using the TTimer stuff. If it were me, I think I’d just PostMessage a custom message to my form from the OnChange event, so the OnChange handler would return immediately. That would allow the PageControl message flow to behave normally. Then in the Message handler for that custom message I would (1) show/start a progress bar form running on a 2nd thread, (2) start the activity which is taking so much time, and (3) when the time consuming activity finishes, shut down the progress bar.

Here’s some code for a threaded progress bar, that I modified from something Peter Below posted years ago. It’s NOT pretty, but users don’t care about that as much as they care about “nothing happening” on the screen.

unit AniMg;
{ Unit for displaying animated progress bar during a lengthy process.
  * Painting of progress is done in a secondary thread, so it updates even during processing
    which doesn't process Windows messages (and therefore doesn't update visible windows).
  * Does NOT call Application.ProcessMessages...so it doesn't alter the order in which the
    application processed messages.
  USAGE:
          //Delays display of the progress form. When this property <> 0, caller must pepper
          //his code with .UpdateVisible calls, or the form will never be displayed.
      AniMgr.DelayBeforeVisible := 3000;
          //If DelayBeforeVisible time has elapsed, displays the progress form & starts thread.
      AniMgr.UpdateVisible;
          //Displays the progress form & starts painting it in a secondary thread.
          //(If DelayBeforeVisible <> 0, sets the form's caption or caption-to-be.)
      AniMgr.Push('Some caption');
          //To change captions without closing/opening the progress bar form...
      AniMgr.Push('Another caption');
          //Close the form
      AniMgr.PopAll;
  NOTES:
  * Do NOT call DisableTaskWindows in this unit!!  It's tempting to do that when the progress
    form is shown, to make it function modally. However, do so at your own risk! Having
    DisableTaskWindows in effect resulted in an AV when we were called from certain routines
    or component's code.
  AUTHOR:
  * Mark Wilsdorf, Flagship Technologies, Inc., www.goflagship.com.
    * Thanks to Peter Below for his original code for painting the progress bar, and his many
      years of providing stellar examples and explanations to the Delphi community.
  DEVELOPMENT:
  * Originally put FAniform.Show/Update on a TTimer delay, so the progress form wouldn't
    display just for a brief instant during quick processes. However, we had to get rid of
    Application.ProcessMessages calls (which caused problems in caller), so the TTimer wouldn't
    fire. Can't make the 2ndary thread do the Show/Update job either, for the same reason:
    Synchronize() won't work because the main thread is occupied in other code, and without
    Application.ProcessMessages calls the Synchronize(Show/Update code) doesn't get called
    until the lengthy main thread code processing finishes. The only solution appears to be:
    have the 2ndary thread be fully responsible for creating and showing/updating the entire
    progress window, entirely via Windows API calls.
}

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, RzLabel, ExtCtrls, RzPanel;

{$I DEFINES.PAS}

type
  T_AniForm = class(TForm)
    RzPanel2: TRzPanel;
    RzLabel1: TRzLabel;
    RzPanel1: TRzPanel;
  public
    r : TRect;
    constructor Create(AOwner: TComponent); override;
  end;

      //Do NOT call DisableTaskWindows in this unit!!
      //We may be called from rtnes or components which attempt to update the UI, resulting
      //in an AV in certain circumstances. This was the result when used with the popular
      //Developer's Express component, ExpressQuantumGrid.

  TAniThread = class(TThread)
  private
    FWnd: HWND;
    FPaintRect: TRect;
    FbkColor, FfgColor: TColor;
    FInterval: integer;
  protected
    procedure Execute; override;
  public
    constructor Create(paintsurface : TWinControl; {Control to paint on }
      paintrect : TRect;          { area for animation bar }
      bkColor, barcolor : TColor; { colors to use }
      interval : integer);        { wait in msecs between paints}
  end;


  TAniMgr = class(TObject)
  private
    FStartTime: DWord;              //=Cardinal. Same as GetTickCount
    FDelayBeforeVisible: cardinal;
    FRefCount: integer;
    FAniThread : TAniThread;
    FAniForm: T_AniForm;
//    procedure SetDelayBeforeVisible(Value: cardinal);
    procedure StopIt;
  public
    procedure Push(const NewCaption: string);
    procedure UpdateVisible;
    //procedure Pop;        Don't need a Pop menthod until we Push/Pop captions...
    procedure PopAll;
        //
        //Delay before form shows. Takes effect w/r/t to first Push() call.
    property DelayBeforeVisible: cardinal read FDelayBeforeVisible write FDelayBeforeVisible;
  end;

function AniMgr: TAniMgr;                //function access


implementation

{$R *.dfm}

var
  _AniMgr : TAniMgr = nil;         //Created privately in Initialization section
      //Do NOT DisableTaskWindows in this unit!!
      //We're called from some rtnes which attempt to update the UI, resulting in an AV.
  //DisabledWindows: pointer = nil;


function AniMgr: TAniMgr;
begin
  if not Assigned(_AniMgr) then
    _AniMgr := TAniMgr.Create;
  Result := _AniMgr;
end;


//---------------------------------------------------------------------------------------------
//                                    TAniMgr
//---------------------------------------------------------------------------------------------


procedure TAniMgr.UpdateVisible;
{ Checks our form's visibility & calls form.Update if appropriate.
  * This rtne implements DelayBeforeVisible handling. }
begin
      //Thd may be terminating...
  if Assigned( FAniThread ) and FAniThread.Terminated then
    exit;

  if Assigned(FAniForm) and
      ( (DelayBeforeVisible = 0) or (GetTickCount - FStartTime > DelayBeforeVisible) ) then begin

    if not Assigned(FAniThread) then
      with FAniForm do begin
        Show;
            //Form.Update processes our paint msgs to paint the form. Do NOT call
            //Application.ProcessMessages here!!  It may disrupt caller's intended message flow.
        Update;             
            //Start painting progress bar on the form
        FAniThread := TAniThread.Create(RzPanel1, r, FAniForm.color, clActiveCaption, 100);
      end
    else
      FAniForm.Update;
  end;
end;


procedure TAniMgr.Push(const NewCaption: string);
{ We don't really Push a stack of captions (though we could)...for now that's not
  important; we just manage the form and RefCount. }
begin
      //Thd may be terminating...
  if Assigned( FAniThread ) and FAniThread.Terminated then
    exit;
  FRefCount := FRefCount + 1;
  if FAniForm = nil then begin
    FAniForm := T_AniForm.Create(nil);
        //If FAniForm was nil this is the first Push() of a series, so get
        //a starting tick count for DelayBeforeShowing management
    FStartTime := GetTickCount;
  end;
  FAniForm.RzLabel1.Caption := NewCaption;
  UpdateVisible;
end;


procedure TAniMgr.StopIt;
begin
  if Assigned( FAniThread ) then begin
    if not FAniThread.Terminated then begin
      FAniThread.Terminate;
      FAniThread.WaitFor;
    end;
  end;
  FreeAndNil(FAniThread);
  FreeAndNil(FAniForm);
end;


//procedure TAniMgr.Pop;
//{ We don't really Pop a stack of captions...for now that's not important; we just
//  decrement the RefCount. }
//begin
//  if FRefCount > 0 then
//    FRefCount := FRefCount - 1;
//  if (FRefCount = 0) then
//    StopIt;
//end;


procedure TAniMgr.PopAll;
begin
  if FRefCount > 0 then try
    StopIt;
  finally
    FRefCount := 0;
  end;
end;


//---------------------------------------------------------------------------------------------
//                                    T_AniForm
//---------------------------------------------------------------------------------------------

constructor T_AniForm.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  r := RzPanel1.ClientRect;
  InflateRect(r, - RzPanel1.BevelWidth, - RzPanel1.BevelWidth);
end;


//---------------------------------------------------------------------------------------------
//                                    TAniThread
//---------------------------------------------------------------------------------------------


constructor TAniThread.Create(paintsurface : TWinControl;
  paintrect : TRect; bkColor, barcolor : TColor; interval : integer);     //BeforePaint: integer);
begin
  inherited Create(True);           //Suspended
  FWnd := paintsurface.Handle;
  FPaintRect := paintrect;
  FbkColor := bkColor;
  FfgColor := barColor;
  FInterval := interval;
  FreeOnterminate := False;       //So we can use WaitFor & know it's dead.
  Resume;
end;


procedure TAniThread.Execute;
var
  image : TBitmap;
  DC : HDC;
  left, right : integer;
  increment : integer;
  imagerect : TRect;
  state : (incRight, incLeft, decLeft, decRight);
begin
  Image := TBitmap.Create;
  try
    with Image do begin
      Width := FPaintRect.Right - FPaintRect.Left;
      Height := FPaintRect.Bottom - FPaintRect.Top;
      imagerect := Rect(0, 0, Width, Height);
    end; { with }
    left := 0;
    right := 0;
    increment := imagerect.right div 50;
      //WAS...    increment := imagerect.right div 50;
    state := Low(State);
    while not Terminated do begin
      with Image.Canvas do begin
        Brush.Color := FbkColor;
        FillRect(imagerect);
        case state of
          incRight: begin
            Inc(right, increment);
            if right > imagerect.right then
            begin
              right := imagerect.right;
              Inc(state);
            end; { if }
          end; { case incRight }

          incLeft: begin
            Inc(left, increment);
            if left >= right then
            begin
              left := right;
              Inc(state);
            end; { if }
          end; { case incLeft }

          decLeft: begin
            Dec(left, increment);
            if left <= 0 then
            begin
              left := 0;
              Inc(state);
            end; { if }
          end; { case decLeft }

          decRight: begin
            Dec(right, increment);
            if right <= 0 then
            begin
              right := 0;
              state := incRight;
            end; { if }
          end; { case decLeft }

        end; { case }
        Brush.Color := FfgColor;
        FillRect(Rect(left, imagerect.top, right, imagerect.bottom));
      end; { with }

      DC := GetDC(FWnd);
      if DC <> 0 then try
        BitBlt(DC,
          FPaintRect.Left,
          FPaintRect.Top,
          imagerect.right,
          imagerect.bottom,
          Image.Canvas.handle,
          0, 0,
          SRCCOPY);
      finally
        ReleaseDC(FWnd, DC);
      end;

      Sleep(FInterval);
    end; { while not Terminated}
  finally
    Image.Free;
  end;
  InvalidateRect(FWnd, nil, True);
end;



initialization

finalization

  if Assigned(_AniMgr) then begin
    _AniMgr.PopAll;
    _AniMgr.Free;
  end;

end.